Level control for carrier-transmission systems



June 19, 1928."-

H. A. 'AFFEL LEVEL CONTROL FOR CARRIER TRANSMISSION SYSTEMS FiledDec. 24, 1925 2 Sheets$heet l [Yecez'ozlmy Elm! Control C(ll'cuit I am 0 Modulator or flenzodzdator ATTORNEY June 19, 1928. I

. H. A. AFFEL LEVEL CONTROL FORCARR-IER TRANSMISSION SYSTEMS Fi ld Dec. .24, 1925 2 Sheets$heet Receiving End fun-u- I N VEN TOR Ell 7w f7??? ATTORNEY Patented June 19, 1928.

UNITED STATES" 1,673,792 PATENT OFFICE.

HERMAN A. AFFEL; MAPLEWOOD, NEW JERSEY, ASSIG NOR TO AMERICAN 'IELE- PHONE AND TELEGRAPH COMPANY, A COBPQRATION OF NEW YORK.

LEVEL CONTROL-FOR CARRIER-TRANSMISSION SYSTEMS.

Application filed December of certain programs, particularly those involving high grade music, renders it necessary totransmit a wide range of; Volume, 1t is desirable to reduce the volume range in transmitting over the line itself to a-relatively narrow range of levels by controlling the input circuit, and to control the output circuit so that the level range may be expanded to its original proportions.

It is the object of this invention to pro- 5 vide, in a carrier system employed in broadcast transmission, means to simultaneously and variably'control the input and the output levels of such a system, which control may be eflected either manually or automatically,

3Q whereby the over-all equivalent of the cir- 'cuit is rendered constant.

This invention will be clearly understood from the following description when read in connection with the attached drawing, of

which Figure 1 shows a form of embodiment of the invention in which the level control is effected manually and by means of a separate control circuit between the trans mitting and the receiving circuits of the system; Fig. 2 is a curve showing the relation between the grid voltage and the gain of a modulator or demodulator; Fig. 3 is another form of embodiment of the invention employing manual control, but utilizing the transmission circuit for effecting the control at the receiving end of the line, and Fig. 4 shows the use of an automatic control system in a transmitting system substantially like Fig. 3.

h In Fig. 1, the line L represents an input circuit having any form of transmitting apparatus connected therewith for picking up the program that is'to be transmitted by means of the carrier system to the distant end of the circuit. This line has connected 24, 1925. Serial No. 77,607."

therewith a level indicator 1 which shows the energy level at the "input of the carrier transmission system. The line L is connected by means of the transformer 2 with the input circuit of the modulator M,, which is preferably of the carrier suppression type shown in the patent to J. R. Carson, No. 1,4el9,382 which issued on 'March 27 1923. 1

This modulator employs the vacuum tubes 3'and 4, the ids of which are connected with the terminals of the secondary winding of the transformer 2, and the plates of which are connected with the terminals of the primary winding'of the transformer 5. The input circuit of the modulator also includes a connection betwe-en tl1e midpoint of the secondary winding of the transformer 2 and the filaments of the tubes 3 and This connection embraces a source of carrier current 6 and a condenser 7 across which is connected the level control device comprising the apparatus within the dotted line 8. This level control device includes a source of direct current voltage 9 for negatively biasing the grids of the tubes and potentiometers 10 and 11. This level control device is connected by means of the control circuit L with the receiving circuit of the carrier system being bridged across the condenser 12 of the demodulator D which will be fully described hereinafter.

The secondary winding of the transformer 5 is connected with the band filter 13, which in turn is connected with an amplifier 1-l-, the connection having bridged across it a level indicator 15. The output side of the amplifier is connected with a band filter 16 which in turn is connected with the line L which connects the transmitting station of the carrier system with the receiving station. The line L, is connected with the receiving circuit by' the transformer 17. This receiving circuit comprises the balanced tubes 18 and' 19, and the circuit is substan tially like the modulating circuit at the transmitting station having a source of carrier frequency 20 connected with it in order current potential 9 provides the grid potential not only for thetubes of the modulator will be seen from the curve shown in Fig. 2, the gain of a modulator or demodulator 1s a function of the grid voltage. This curve shows that as the grid voltage increases, the gain of a modulator or demodulator is like wise increased, and a decrease of grid voltage I produces an opposite effect upon the modu- ,lator'or demodulator. The circuit of the level control device 8 is so arranged that a movement of the potentiometer 10-11 in the direction for increasing the grid poten-' tial of the modulator M at the transmitting end effects also a variation in the gridpotential of the dei iodulator D at the receiving end, the latter change being in opposite di- "rection tothe former. It is, of course, desirable that the normal grid voltage upon the tubes shall be such that the variation up- ,wards and downwards of the said voltage may be made along-the straight part of the curve shown in Fig. 2.

In the operation of the system, the operator watches the level indicator 1 during the time in which the audio frequencies representing the program are applied by the line L to the modulator M,.' The carrier frequency ofthe source 6 which will be modulated by the audio frequencies will be impressed upon the circuit containing the band filter 13, the amplifier 14 and the band filter 16. J1 single side band would be passed by v the filters 13 and 16 and amplified by the' amplifier 14, and after transmission over the line L.,, would be detected by the demodulating circuit D If the level of the input cir-. cuitlD increases above the desired point, the operator'adjusts the potentiometer ilO- ll in order to decrease the gain produced by the modulator M and to increase the gain produced by the demodulator D,. In this way, the transmission equivalcntmf the line a, a

: whole'is maintained constanti The arrangement shown in Fig. 3, in which the same reference symbols have been used, as in Fig. 1, is also a manually controlled system, but differs from the arrangement shown in Fig. 1 in that it does not require a separate control circuit such as L of Fig. 1

to effect the regulation of the grid voltage of the demodulator D atthe receiving .end

' vof the system. The output side of the transmitting circuit is connected with the branch transmission line TL to which a plurality of similar transmitting circuits of other channels may be connected. This transmitting branch has in it a. high pass filter 22 in order to select the carrier channels and to discriminate against low frequency signals 'which also may be sent simultaneously over the line L lln similar manner, atthe receiving end of the system, the branch receiving line RL isconnected with the line L and has in it a high pass filter 25. The input circuit of the modulator M, has a normal grid biasing battery 30, and similarly the demodulator 1),, a battery 31. The level control device consisting of the battery 9 and a potentiometer 29 is arranged so that its potential will vary in opposite ways the grid potential of the modulator tubes 3 and 4, and the demodulating tubes 18 and 19.

This variation of the grid potential is efiec tive over the branch circuits 34c.and 35 connected by the line L Transmission of the unmodulated carrier from the sources 6 and 20 is prevented composite sets C and OX which permit pg the employment of the.

the passage of the direct current potentials of the grid biasing battery 9, but suppress the high frequency carrier.

The method of operation is substantially the same as in Fig. 1, the operator watching the level indicator 1 and Varying the potentiometer 29 to efi'ect a reduction of the gain of the modulator M and an increase in the gain of the demodulator 1), whenever the level of the line L increases above a termined amount. lit the level of the line L, falls, the operator would, of course, produce an opposite effect, namely, increase the gain of the modulator M and decrease that of the demodulator D The arrangement shown in Fig. 4 is structurally similar to that shown in Fig. 3, excepting that by means of the elements 40 to 43, inclusive, the level control is automatically effected. That is to say, as the level rises and falls in the line L 'above or below a predetermined limit, a small portion of this energy is taken off by the branch circuit with which the regulation isconnected, and is impressed by the transformer 40 upon the rectifier- 41.. The magnitude of the rectified energy will, of course, depend upon the level of the voltage in the line L,. This rectified will be applied to the input circuits of the modulator andthe' demodulator across the rederesponsive level control.

'line L through'the composite sets GK, and

0K, in the manner in which the control voltage was transmitted in the arrangement of Fi 3.

'Ihe low pass filter 42 will have to be adjusted in transmission characteristics so that the resultant gain control is neither too fast, that is to say, it should not follow all of the rapid fluctuations of the audio energy, or too slow, that is, not providing sufiiciently If desirable, time lag circuits may be introduced in various parts of the arrangements to efiect some improvement-in its practical operation. It is possible to have the control channel transmitted at carrier frequency from the sending to the receiving end, and it is furthermore possible to have this control channel provided by the carrier current itself, since the effective gain of the modulator or demoducarrier energy supplied. In such a case, instead of having a constant carrier supplied,

as illustrated in the figure, thegrid potential might be made constant and the carrier supply transmitted to both terminals and caused to varyproperly in relation to the input of the voice energy.

While this invention has been disclosed as embodied in certain forms, it is capable of embodiment in other and different forms without departing ,from the spirit and scope of the appended claims.

What is claimed is:

1. In a signaling systemin which a high frequency wave is modulated by a signal, the combination with a transmission circuit of a signal input circuit, a signal output circuit, a modulating circuit, havinga source of carrier current connected therewith, effectively connecting saidsignal input circuit with said transmission circuit, a demodulating circuit, likewise having a source of carrier current, effectively connecting said transmission circuit with said signal output circuit, and means connected with said signal'input circuit and with said modulatingand demodulating circuits and regulated by the strength of the input signal to control the energy level at the input and output ends of said transmission circuit, thereby maintaining constant the over-all equivalent of said system.

2. In. a signaling system in which a high frequency wave is modulated by a signal, the combination with a transmission circuit of a signal input circuit, a signal output circuit, a modulating circuit interposed between said input circuit and'said transmission circuit to modulate a ca rier current by said signal and to impress it upon sald transmisslon an cuit, a demodulating circuit interposed between said transmission circuit and said output circuit to dcmodulate the modulated carrier and to impress the resulting signal upon said'output circuit and means controlled by the current of the signal input circuit to control the magnitude of the gain produced by said modulatorand demodulator, thereby automatically maintaining constant the overall transmission.equivalent of said system.

3. In a signaling system in which a high frequency wave is modulated by a signal, the combination with a transmission circuit of a signal input circuit, a signal output circuit, a modulating circuit interposed between said input circuit and said transmission circuit to modulate a-carrier current by said signal and to impress it upon said transmission circuit, a demodulating circuit inand said output circuit to demodulate the modulated carrier and to impress the resultingsignal upon said output circuit, a rectificr'having its input side connected with the said input circuit and its output side connected with said modulating and demodulating circuits, the latter connection including.

a low pass filter to smooth out the unidirectional current resulting from the rectification of the signal potential applied to said rectifier, thereby automatically maintaining constant the over-all transmission equivalent of said system.

4 In a signaling system in which a high frequency wave is modulated by a signal, the combination with a transmission circuit of a signal input circuit, a signal'output' circuit, a vacuum tube modulator interposed between-said input circuit and said transmission circuit having a source of carrier oscillations adapted to be modulated by the singal, a vacuum tube demodulator interposed between the said transmission circuit and the said output circuit adapted to delzlOdllltlte the said'modulated carrier oscillations, and rectifying means having its input side connected with said input circuit and its output side connected with said vacuum tube modulator and demodulator adapted to vary the grid potential of the vacuum tubes in accordance with the variations in strength of the input signal.

5. In a signaling system in which a high frequency wave is modulated by asignal, the combination with a transmission circuit of a signal input circuit, a signal output circuit, a modulating circuit connecting said input circuit with said transmission circuit, a demodulating circuit connecting said transmission circuit with said output circuit, and means responsive to the strength of the input signal to automatically maintain constantthe over-all transmission equivalent of' the system.

6. In a signaling system in which a high frequency wave is modulated by a signal, the combination with a transmission circuit of asignal input circuit, a signal output circuit, a modulating-circuit connecting said input circuit with saidtransmission circuit, a demodulating circuit connecting the said transmission circuitwith the said output circuit, and a rectifier having its input'side connected with the signal input circuit and having its output side connected With said modulating circuit and said demodulating circuit, the said latter connection including a 10W pass filter to smooth out the rectified voltage.

7. In a slgnaling system in which a high frequency wave 15 modulated by a slgnal, the comblnation With a transmission circuit ofa signal input circuit, a signal output cirsaid variations corresponding to changes in the energy level of the signal inputc ircuit.-

In testimony whereof, I have signed my name to this specification this 23rd day of December, 1925. V

' HERMAN A. AFFEL. v 

